Designing an outer surface of a composite part

ABSTRACT

The disclosure notably relates to a computer-implemented method for designing an outer surface of a composite part manufactured by molding a stack of material layers. The method includes defining constant offset surfaces, a constant offset surface being a respective part of the outer surface which is to have a constant offset value relative to the reference surface, the constant offset value of a respective constant offset surface corresponding to the sum of the thicknesses of the material layers below the respective constant offset surface, and determining a final surface that corresponds to a tangent continuous connection of the constant offset surfaces. This provides an improved solution for designing an outer surface of a composite part.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of, and claims thebenefit of priority under 35 U.S.C. § 120 from, U.S. application Ser.No. 15/386,681, filed Dec. 21, 2016, which claims the benefit ofpriority under 35 U.S.C. § 119 or 365 to European Application No.15307098.2, filed Dec. 22, 2015. The entire contents of the aboveapplication(s) are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to the field of computer programs and systems, andmore specifically to a method, system and program for designing an outersurface of a composite part.

BACKGROUND

A number of systems and programs are offered on the market for thedesign, the engineering and the manufacturing of objects. CAD is anacronym for Computer-Aided Design, e.g. it relates to software solutionsfor designing an object. CAE is an acronym for Computer-AidedEngineering, e.g. it relates to software solutions for simulating thephysical behavior of a future product. CAM is an acronym forComputer-Aided Manufacturing, e.g. it relates to software solutions fordefining manufacturing processes and operations. In such computer-aideddesign systems, the graphical user interface plays an important role asregards the efficiency of the technique. These techniques may beembedded within Product Lifecycle Management (PLM) systems. PLM refersto a business strategy that helps companies to share product data, applycommon processes, and leverage corporate knowledge for the developmentof products from conception to the end of their life, across the conceptof extended enterprise. The PLM solutions provided by Dassault Systèmes(under the trademarks CATIA, ENOVIA and DELMIA) provide an EngineeringHub, which organizes product engineering knowledge, a Manufacturing Hub,which manages manufacturing engineering knowledge, and an Enterprise Hubwhich enables enterprise integrations and connections into both theEngineering and Manufacturing Hubs. All together the system delivers anopen object model linking products, processes, resources to enabledynamic, knowledge-based product creation and decision support thatdrives optimized product definition, manufacturing preparation,production and service.

In this context, some solutions allow the design of composite parts, asthese are mechanical parts more and more at use in many industries.However, existing solutions are not fully satisfactory. Notably, somesolutions provide an approximate representation of the composite part,but not accurate enough for the manufacturing end to use it as such, oreven for simulations performed on such a representation to be relevantenough. Other solutions provide general tools for local designs, thatcan be theoretically used for composite material design as well. Suchsolutions can be used to repair non-accurate designs of composite parts,or even design such composite parts from scratch, but they require manyuser interventions and are thus too inefficient from the point of viewof ergonomics.

Within this context, there is still a need for an improved solution fordesigning an outer surface of a composite part.

SUMMARY OF THE INVENTION

It is therefore provided a computer-implemented method for designing anouter surface of a composite part manufactured by molding a stack ofmaterial layers. Each material layer has a respective thickness and arespective ply boundary. The method comprises providing a referencesurface and a ply map that represents the stack of material layersbetween the reference surface and the outer surface. The ply map isdefined as a list of contours. The contours are defined on the referencesurface. Each contour is associated to the ply boundary of a respectivematerial layer. The method also comprises defining constant offsetsurfaces. A constant offset surface is a respective part of the outersurface which is to have a constant offset value relative to thereference surface. The constant offset value of a respective constantoffset surface corresponds to the sum of the thicknesses of the materiallayers below the respective constant offset surface. And the methodcomprises determining a final surface that corresponds to a tangentcontinuous connection of the constant offset surfaces.

Because the final surface corresponds to a tangent continuous connectionof such specific constant offset surfaces, the final surface representsthe composite part realistically and/or with relatively high accuracy,and the method thereby provides useful specifications for later relevantsimulation(s) and/or production(s), such as the production of a mold forthe manufacturing of the composite part. Thanks to it being simple andhighly automatable, the method provides relatively high ergonomics tothe user.

The method may comprise one or more of the following:

-   -   the constant offset surfaces each have a respective boundary        line parallel to the ply boundary of a respective material        layer, the distance between the boundary line of a respective        constant offset surface and the ply boundary of the respective        material layer being larger than or substantially equal to a        step width between the respective constant offset surface and        the next and lower constant offset surface;    -   each transition surface corresponds to a convolution product        between a function ƒ* defining a continuous connection of the        constant offset surfaces that interpolates the ply boundaries        and a convolution kernel ϕ defined for x∈        ³ and of the type ϕ(x)=0 if ∥x∥>w, where ∥x∥ is the Euclidean        norm of x and w>0 is the convolution radius;    -   the convolution kernel is of the type

${\phi(x)} = {\frac{1}{k}\left( {w^{2} - {x}^{2}} \right)^{3}}$if ∥x∥≤w, where k=∫_({∥x∥<w})(w²−∥x∥²)³dx;

-   -   function ƒ* is defined over a mesh fitting the reference surface        and having faces, ƒ* being affine over each face of the mesh,        the contours of the ply map being formed by respective connected        edges of the mesh, the respective edges being mapped in        bijection by ƒ* onto the ply boundaries;    -   other respective connected edges of the mesh are mapped in        bijection by ƒ* onto the boundary lines of the constant offset        surfaces;    -   other respective connected edges of the mesh are mapped in        bijection by ƒ* onto junction lines, a junction line joining the        respective boundary lines of two consecutive constant offset        surfaces; and/or    -   function ƒ* corresponds to a minimum of an energy that penalizes        the image of ƒ* over each face of the mesh deviating from a        constant value.

It is further provided an outer surface of a composite part manufacturedby molding a stack of material layers, each material layer having arespective thickness and a respective ply boundary, the outer surfacebeing obtainable according to the method.

It is further provided a process for producing a mold adapted for themanufacturing of a composite part by molding a stack of material layers,each material layer having a respective thickness and a respective plyboundary, the process comprising receiving specifications of the outersurface and manufacturing a mold that physically corresponds to thereceived outer surface.

It is further provided a computer program comprising instructions forperforming the method.

It is further provided a computer readable storage medium havingrecorded thereon the computer program and/or the outer surface.

It is further provided a system comprising a processor coupled to amemory and a graphical user interface, the memory having recordedthereon the computer program.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way ofnon-limiting example, and in reference to the accompanying drawings,where:

FIG. 1 shows a flowchart of an example of the method;

FIG. 2 shows an example of a graphical user interface of the system;

FIG. 3 shows an example of the system; and

FIGS. 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21,22, 23, 24, 25, 26, 27, 28, 29, 30, 31A, 31B, 32 and 33 illustrate themethod.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the flowchart of FIG. 1 which shows an example, it isproposed a computer-implemented method for designing an outer surface ofa composite part.

As known per se, a composite part may be traditionally manufacturedthrough a Resin Transfer Molding process (RTM in the following) or thelike, e.g. by molding a stack of material layers (i.e. layers ofmaterial(s) piled up one over the other, stacked in a mold having arespective stacking direction, e.g. a direction representative of theintegral of the local pressures, e.g. considered indifferently before orafter the molding—which can be referred to as the local stackingdirection—exerted by the mold on the stack of material layers, and thenmolded together), e.g. each layer being made of a same material or acombination of different materials, e.g. different layers being madeeither of a same material or same combination of materials, or eitherdifferent materials or different combinations of materials). The moldingmaterial can be any adapted material, such as resin, e.g. which isinjected/casted e.g. under liquid form, e.g. with a high temperature,and then e.g. cured/hardened. Each material layer has a respectivethickness and a respective ply boundary. The thickness is a valuerepresentative of the height of the layer with respect to the stackingdirection. The thickness may be, for each layer, and independently fromthe other layers, constant or variable (the following discussions beingillustrative of the constant case, adaptation to the variable case beingcontemplatable by the skilled person). The ply boundary is the upperhorizontal edge of the layer (with respect to the stacking direction),before the molding. Generally, the ply boundary is the edge of the layerin contact with the mold during the molding, and it is—e.g.slightly—flattened upon the stacking, due to the pressure, before themolding (thanks to the material layers being—e.g. slightly—deformable).In examples of the method, this flattening is captured by dedicatedspecifications. In case materials allowing no deformation are at use,such specific examples of the method may simply be adapted byaccordingly adding corresponding quantities to the relevant parameters.

Now, the method is for designing a computer-implemented representationof an outer surface of such a composite part. The outer surface is atleast a part of the envelope of the composite part (as resulting fromthe molding), possibly forming the whole envelope of the part togetherwith the reference surface or with a symmetrically designed other outersurface. For example, the outer surface designed by the method of FIG. 1is the upper surface of the composite part, with respect to the stackingdirection. But it can alternatively be the lower surface, depending onthe convention retained for the direction, being noted that thecomposite part can also be made of a stacking in both directions (thefollowing discussions being illustrative of the unique stackingdirection case). Now, by construction, such an outer surface of thecomposite part corresponds, in terms of geometry, to the inner surfaceof the upper matrix of a mold adapted for the manufacturing of thecomposite part (the inner surface of the lower matrix of the moldcorresponding to the reference surface mentioned hereunder or to anotherouter surface—the discussions herein applying symmetrically in such acase—in case the stacking is performed in two directions).

Thus, the outer surface designed by the method can be used to produce amold adapted for the manufacturing of the composite part whose outersurface was designed. For example, a designer may design the outersurface according to the method, and then the designer or another person(possibly the mold producer) may fix specifications thereof (e.g. bystoring specifications of the outer surface on persistent memory, and/orby adding other specifications), thereby handling a modeled surfacehaving a specific structure (i.e. specific geometry provided by thetangent continuity and/or specific topology, provided by the surfaceconnections and yet in examples by the underlying meshing, discussedlater). Then another user (e.g. the so-called “mold producer”) and/or acomputer may receive said specifications and manufacture (with adaptedmachinery) a mold based on such specifications. Notably, at least aportion of the mold (e.g. upper matrix, notably corresponding innersurface) physically corresponds to the received outer surface (i.e. thespecifications are followed directly—thus optimizing the mold productiontime—and translated into material that forms said part of the mold, thefinal physical geometry corresponding to the virtual geometry designedby the method). As understood by the skilled person, the end user (i.e.the mold producer) possesses tools including software that are adaptedfor reading the format of the received outer surface specifications andmanufacturing the mold. Once the mold is obtained, the composite partcan in turn be manufactured (as explained above), industrially, forexample in series.

The method is computer-implemented. This means that the steps (orsubstantially all the steps) of the method are executed by at least onecomputer, or any system alike. Thus, steps of the method are performedby the computer, possibly fully automatically, or, semi-automatically.In examples, the triggering of at least some of the steps of the methodmay be performed through user-computer interaction. The level ofuser-computer interaction required may depend on the level of automatismforeseen and put in balance with the need to implement user's wishes. Inexamples, this level may be user-defined and/or pre-defined. Forinstance, the providing S10 and/or the defining S20 may be performed viauser-interaction (e.g. the providing S10 amounting to a step-by-stepdesign, e.g. from scratch, or to the retrieval from persistent memory ofearlier-defined specifications by launching a dedicated functionality).The determining S50 (and/or S40) may be performed fully automatically(e.g. by a direct implementation/computation of the mathematicsexplained later), so as to decrease the burden put on the user andimprove ergonomics.

A typical example of computer-implementation of the method is to performthe method with a system adapted for this purpose. The system maycomprise a processor coupled to a memory and a graphical user interface(GUI), the memory having recorded thereon a computer program comprisinginstructions for performing the method. The memory may also store adatabase. The memory is any hardware adapted for such storage, possiblycomprising several physical distinct parts (e.g. one for the program,and possibly one for the database).

The method generally manipulates modeled objects. A modeled object isany object defined by data stored e.g. in the database. By extension,the expression “modeled object” designates the data itself. According tothe type of the system, the modeled objects may be defined by differentkinds of data. The system may indeed be any combination of a CAD system,a CAE system, a CAM system, a PDM system and/or a PLM system. In thosedifferent systems, modeled objects are defined by corresponding data.One may accordingly speak of CAD object, PLM object, PDM object, CAEobject, CAM object, CAD data, PLM data, PDM data, CAM data, CAE data.However, these systems are not exclusive one of the other, as a modeledobject may be defined by data corresponding to any combination of thesesystems. A system may thus well be both a CAD and PLM system, as will beapparent from the definitions of such systems provided below.

By CAD system, it is additionally meant any system adapted at least fordesigning a modeled object on the basis of a graphical representation ofthe modeled object, such as CATIA. In this case, the data defining amodeled object comprise data allowing the representation of the modeledobject. A CAD system may for example provide a representation of CADmodeled objects using edges or lines, in certain cases with faces orsurfaces. Lines, edges, or surfaces may be represented in variousmanners, e.g. non-uniform rational B-splines (NURBS). Specifically, aCAD file contains specifications, from which geometry may be generated,which in turn allows for a representation to be generated.Specifications of a modeled object may be stored in a single CAD file ormultiple ones. The typical size of a file representing a modeled objectin a CAD system is in the range of one Megabyte per part. And a modeledobject may typically be an assembly of thousands of parts.

In the context of CAD, a modeled object may typically be a 3D modeledobject, e.g. representing a product such as a composite part or a moldadapted for manufacturing thereof. By “3D modeled object”, it is meantany object which is modeled by data allowing its 3D representation. A 3Drepresentation allows the viewing of the part from all angles. Forexample, a 3D modeled object, when 3D represented, may be handled andturned around any of its axes, or around any axis in the screen on whichthe representation is displayed. This notably excludes 2D icons, whichare not 3D modeled. The display of a 3D representation facilitatesdesign (i.e. increases the speed at which designers statisticallyaccomplish their task). This speeds up the manufacturing process in theindustry, as the design of the products is part of the manufacturingprocess.

The 3D modeled object may represent the geometry of a product to bemanufactured in the real world subsequent to the completion of itsvirtual design with for instance a CAD software solution or CAD system,i.e. a composite part or a mold adapted for the manufacturing thereof. ACAD software solution allows the design of products in various andunlimited industrial fields, including: aerospace, architecture,construction, consumer goods, high-tech devices, industrial equipment,transportation, marine, and/or offshore oil/gas production ortransportation. The composite part designed by the method may thusrepresent an industrial product which may be any composite part, such asa composite part of a terrestrial vehicle (including e.g. car and lighttruck equipment, racing cars, motorcycles, truck and motor equipment,trucks and buses, trains equipment), a composite part of an aerialvehicle (including e.g. airframe equipment, aerospace equipment,propulsion equipment, defense products, airline equipment, spaceequipment), a composite part of a naval vehicle (including e.g. navyequipment, commercial ships, offshore equipment, yachts and workboats,marine equipment), or a composite part of an architectural product (e.g.house, building structural equipment).

By PLM system, it is additionally meant any system adapted for themanagement of a modeled object representing a physical manufacturedproduct (or product to be manufactured). In a PLM system, a modeledobject is thus defined by data suitable for the manufacturing of aphysical object. These may typically be dimension values and/ortolerance values. For a correct manufacturing of an object, it is indeedbetter to have such values.

By CAM solution, it is additionally meant any solution, software ofhardware, adapted for managing the manufacturing data of a product. Themanufacturing data generally includes data related to the product tomanufacture, the manufacturing process and the required resources. A CAMsolution is used to plan and optimize the whole manufacturing process ofa product. For instance, it can provide the CAM users with informationon the feasibility, the duration of a manufacturing process or thenumber of resources, such as specific robots, that may be used at aspecific step of the manufacturing process; and thus allowing decisionon management or required investment. CAM is a subsequent process aftera CAD process and potential CAE process. Such CAM solutions are providedby Dassault Systèmes under the trademark DELMIA®.

By CAE solution, it is additionally meant any solution, software ofhardware, adapted for the analysis of the physical behavior of modeledobject. A well-known and widely used CAE technique is the Finite ElementMethod (FEM) which typically involves a division of a modeled objectinto elements which physical behaviors can be computed and simulatedthrough equations. Such CAE solutions are provided by Dassault Systèmesunder the trademark SIMULIA®. Another growing CAE technique involves themodeling and analysis of complex systems composed a plurality componentsfrom different fields of physics without CAD geometry data. CAEsolutions allows the simulation and thus the optimization, theimprovement and the validation of products to manufacture. Such CAEsolutions are provided by Dassault Systèmes under the trademark DYMOLA®.

PDM stands for Product Data Management. By PDM solution, it is meant anysolution, software of hardware, adapted for managing all types of datarelated to a particular product. A PDM solution may be used by allactors involved in the lifecycle of a product: primarily engineers butalso including project managers, finance people, sales people andbuyers. A PDM solution is generally based on a product-orienteddatabase. It allows the actors to share consistent data on theirproducts and therefore prevents actors from using divergent data. SuchPDM solutions are provided by Dassault Systèmes under the trademarkENOVIA®.

FIG. 2 shows an example of the GUI of the system, wherein the system isa CAD system.

The GUI 2100 may be a typical CAD-like interface, having standard menubars 2110, 2120, as well as bottom and side toolbars 2140, 2150. Suchmenu- and toolbars contain a set of user-selectable icons, each iconbeing associated with one or more operations or functions, as known inthe art. Some of these icons are associated with software tools, adaptedfor editing and/or working on the 3D modeled object 2000 displayed inthe GUI 2100. The software tools may be grouped into workbenches. Eachworkbench comprises a subset of software tools. In particular, one ofthe workbenches is an edition workbench, suitable for editinggeometrical features of the modeled product 2000. In operation, adesigner may for example pre-select a part of the object 2000 and theninitiate an operation (e.g. change the dimension, color, etc.) or editgeometrical constraints by selecting an appropriate icon. For example,typical CAD operations are the modeling of the punching or the foldingof the 3D modeled object displayed on the screen. The GUI may forexample display data 2500 related to the displayed product 2000. In theexample of FIG. 2, the data 2500, displayed as a “feature tree”, andtheir 3D representation 2000 pertain to a brake assembly including brakecaliper and disc. The GUI may further show various types of graphictools 2130, 2070, 2080 for example for facilitating 3D orientation ofthe object, for triggering a simulation of an operation of an editedproduct or render various attributes of the displayed product 2000. Acursor 2060 may be controlled by a haptic device to allow the user tointeract with the graphic tools.

FIG. 3 shows an example of the system, wherein the system is a clientcomputer system, e.g. a workstation of a user.

The client computer of the example comprises a central processing unit(CPU) 1010 connected to an internal communication BUS 1000, a randomaccess memory (RAM) 1070 also connected to the BUS. The client computeris further provided with a graphical processing unit (GPU) 1110 which isassociated with a video random access memory 1100 connected to the BUS.Video RAM 1100 is also known in the art as frame buffer. A mass storagedevice controller 1020 manages accesses to a mass memory device, such ashard drive 1030. Mass memory devices suitable for tangibly embodyingcomputer program instructions and data include all forms of nonvolatilememory, including by way of example semiconductor memory devices, suchas EPROM, EEPROM, and flash memory devices; magnetic disks such asinternal hard disks and removable disks; magneto-optical disks; andCD-ROM disks 1040. Any of the foregoing may be supplemented by, orincorporated in, specially designed ASICs (application-specificintegrated circuits). A network adapter 1050 manages accesses to anetwork 1060. The client computer may also include a haptic device 1090such as cursor control device, a keyboard or the like. A cursor controldevice is used in the client computer to permit the user to selectivelyposition a cursor at any desired location on display 1080. In addition,the cursor control device allows the user to select various commands,and input control signals. The cursor control device includes a numberof signal generation devices for input control signals to system.Typically, a cursor control device may be a mouse, the button of themouse being used to generate the signals. Alternatively or additionally,the client computer system may comprise a sensitive pad, and/or asensitive screen.

The computer program may comprise instructions executable by a computer,the instructions comprising means for causing the above system toperform the method. The program may be recordable on any data storagemedium, including the memory of the system. The program may for examplebe implemented in digital electronic circuitry, or in computer hardware,firmware, software, or in combinations of them. The program may beimplemented as an apparatus, for example a product tangibly embodied ina machine-readable storage device for execution by a programmableprocessor. Method steps may be performed by a programmable processorexecuting a program of instructions to perform functions of the methodby operating on input data and generating output. The processor may thusbe programmable and coupled to receive data and instructions from, andto transmit data and instructions to, a data storage system, at leastone input device, and at least one output device. The applicationprogram may be implemented in a high-level procedural or object-orientedprogramming language, or in assembly or machine language if desired. Inany case, the language may be a compiled or interpreted language. Theprogram may be a full installation program or an update program.Application of the program on the system results in any case ininstructions for performing the method.

“Designing a 3D modeled object” designates any action or series ofactions which is at least part of a process of elaborating a 3D modeledobject. Thus, the method may comprise creating the 3D modeled objectfrom scratch. Alternatively, the method may comprise providing a 3Dmodeled object previously created, and then modifying the 3D modeledobject.

The method may be included in a manufacturing process, which maycomprise, after performing the method, producing a physical productcorresponding to the modeled object (e.g. a mold and/or a compositepart, as discussed earlier). In any case, the modeled object designed bythe method may represent a manufacturing object. The modeled object maythus be a modeled solid (i.e. a modeled object that represents a solid).Because the method improves the design of the modeled object, the methodalso improves the manufacturing of a product and thus increasesproductivity of the manufacturing process.

The example of FIG. 1 is now discussed in more details.

The method of the example comprises the providing S10 of inputspecifications, very classical in the design of composite parts. Asmentioned earlier, these input specifications may be designed fromscratch in any way (notably, with any software, e.g. belonging or not tothe same software suite as the software underlying the execution of themethod of FIG. 1), or retrieved from memory in any way (e.g. withpossibly a later user-finalization before execution of the method ofFIG. 1), for example upon the user launching a dedicated softwarefunctionality and selecting an input (e.g. from a library). These inputspecifications comprise a reference surface and a ply map defined withrespect to the reference surface. Now, from the point of view of themethod, these input specifications fully characterize the composite partinitially, such that the method of FIG. 1 may run based exclusively onthis input (but in examples together with additional data, as mentionedlater).

The reference surface designates any data that define a 3D surface, suchas a NURBS, a B-Rep or any parameterized surface. The method is fordesigning an outer surface of a composite part, and as a constraint tothe method, the outer surface is eventually continuously connected (e.g.tangent continuous connection or C1 connection) to the reference surface(or part of such connection), the two surfaces thereby representing theenvelope of a solid portion of the composite part. As such, thereference surface may represent the lower or upper surface of the lowestmaterial layer, or the inner surface of the lower matrix of the mold, or(e.g. if the composite part is defined as a stacking in two directions)the lower or upper or central surface of a central material layer.

Now, the reference surface is associated to a ply map, which is bydefinition data that represent the stack of material layers between thereference surface and the outer surface (before the molding, and thusthe potential flattening mentioned earlier). The portion of thecomposite part covered by the outer surface designed by the method thusincludes such a stack of material layers. The ply map is defined as alist (i.e. ordered set, e.g. from the referenced surface to the top,i.e. along the stacking direction) of contours (i.e. closed curve orcurves) defined on the reference surface. The ply map represents thematerial layers, which means that each contour is associated to the plyboundary of a respective material layer (i.e. it corresponds to such aply boundary, such that each contour is a projection along therespective normal directions of the reference surface of the theoreticply boundary). Each contour corresponds to a ply boundary (i.e. edge),which is by construction physically distant from the reference surfaceby the sum of thicknesses of material layers associated to all contoursbefore it (the thickness associated to the contour under considerationbeing included in the sum). The contours are thus each associated to athickness (the association being more that an abstract correspondence,but a data association, e.g. via pointer(s) and/or link(s) that allowretrieving the relevant sum of thicknesses for each contour), the sum ofall these information representing the stack of material layers.Similarly, geometry (e.g. curves/lines or mesh edges) defined on or withrespect to the reference surface may all be associated (with pointingdata or abstractly) to geometry that belongs to the final surface (i.e.the outer surface being designed). The following discussions mayindifferently refer to the geometry on the reference surface or to theassociated final surface geometry with the same wording (for the sake ofconcision).

Now, the contours can be (for mechanical reasons) smaller and smaller, acontour being included (strictly or not) in the previous contour of thelist (i.e. the contours thereby defining one or several geometricalpyramid(s) from the reference surface to the “top”, and corresponding“steps” of the ply map, a “step” designating the restriction band of thereference surface delimited by two consecutive contours). In case thisinclusion transitivity (along the ordering) is not respected, during themanufacturing resin is inserted between two layers separated by exactlyone intermediary layer (a layer i and a layer i+2), against theintermediary layer (layer i+1) separating the two layers. In such acase, the problem can be rewritten and the outer surface can be designedin the same way, for example provided that the method comprisesre-defining (e.g. automatically) the ply map, e.g. considering that thecontour of the intermediary layer is the same as the one of thepreceding layer or the one of the next layer, or somewhere between, e.g.an average of the two contours), or provided that the method disregardsthe intermediary layer and simply adds its thickness to the next layeror to the previous layer (such adaptation being an implementationdetail). It is noted that the ply map can be provided as such a list ofcontours, or as any other form of data from which the list of contourscan be retrieved (the distinction not being considered in thefollowing).

The method of the example then comprises defining S20 constant offsetsurfaces. A constant offset surface is by definition a respective partof the outer surface which is to have a constant offset value relativeto the reference surface. In other words, a constant offset surface is aconstraint imposed to the method (specifically, a constraint taken intoaccount at the later determining S50). This constraint can be defined inany way, provided that it leads to the respective parts of the outersurface having a constant offset value relative to the reference surface(e.g. the offset being measured with respect to a normal vector of thereference surface in the stacking direction). Composite parts indeedcorrespond to different plateaus unitarily formed (e.g. with resinsbinding the plateaus together and to the reference surface), dependingon the configuration of the stack of material layers. And the methodcaptures this information by imposing such a constraint, as when theconstant offset zones are connected with a tangent continuous surface,this accurately represents a plausible composite part (i.e. that can bemanufactured as such). In specific, the constant offset value of arespective constant offset surface corresponds, e.g. it is equal(substantially, as the thicknesses may be constant but alternatively notperfectly constant and thus regularized by the molding material, e.g.resin, and/or a layer of molding material with a negligible thicknessmay be superimposed over the top layer(s) of the stack of materiallayers, this negligible thickness being predetermined, e.g. constant oras a function of the parameters/configuration of the contemplatedmolding process), to the sum of the thicknesses of the material layersbelow the respective constant offset surface.

The definition S20 of constant offset surfaces can be performed in anyway, e.g. fully automatically (e.g. based on an automatic analysis ofthe characteristics of the input specifications) and/orvia-user-interaction (e.g. possibly including also semi-automaticdefinition). The example of FIG. 1 shows an example of suchuser-interaction to perform the definition S20, where the methodcomprises user-interaction to define concurrently (the order having noimportance in the case of the example) at S22 a so-called “convolutionradius” w (which performs for designated steps of the reference surfacerespective restrictions—the boundary lines computed at S24 bounding suchrestrictions—that are to correspond to a constant offset zone) and atS26 junction lines (which designate those steps, any other way todesignate those steps being however possible, such as for example thosesteps being predetermined to the method, as designated to theapplication user, the junction lines forming thereby merely optionaladditional data), the term “defining” designating both the user settingvalues from scratch or the user validating/completing/correctingpre-computed values. It is noted that value w is called “convolutionradius” (here already) because the same value may be in examples used ina convolution product (as discussed later), so that the result of themethod is particularly accurate (although this is not necessary).

In the example, the constant offset zones are defined as restrictions ofcorresponding respective steps of the ply map. Now, as typical for acomposite part, one category of steps (“first” steps in the following)of the ply map may be meant (e.g. by a user having designed the ply map)for a constant offset surface (that is, a plateau), while anothercategory of steps (i.e. the other steps, that is, those between thefirst steps, “second” steps in the following) define so-called“transition zones” between the constant offset zones that are meant toreceive molding material that performs a junction between the constantoffset zones (i.e. zones of the outer surface which are never constant,at least for a significant width, e.g. more than the half or the quarterof the width of the corresponding step when projected on the referencesurface, these zones being referred to as “transition surfaces” or again“transition zones”). Typically, and at least statistically (e.g. for atleast 75% of the units considered in a case), first steps are relativelywide, while second steps are narrow and define successive steps (i.e.“stairs”) between the first steps (before the molding smoothenseverything).

This configuration can be specified fully by the user, or with computerautomatic assistance (fully or semi-automatically), or predetermined tothe method (i.e. the steps are pre-provided with a “first” or “second”type, and possibly with the junction lines as well, instead of having todefine them at S26). In an example, the method is executed for a ply mapwhere the smallest width of a first step (i.e. average or minimumdistance between the two contours of said step), e.g. in the 75% highesttier of first steps with respect to the width, is larger than (e.g. atleast once and a half) the largest width of a second step (the widthbeing defined in the same way), e.g. in the 75% lowest tier of secondsteps with respect to the width. The system can in such a case comparedistances and detect steps meant for constant offset zones and/or stepsmeant for transition zones automatically, and the user can potentiallyperform validations and/or corrections if necessary. This detection is amatter of implementation and not the subject of the present discussions.

In the example, first steps and second steps may be specified orcompleted by defining S26 junction lines, which are lines that eachcorrespond to a respective curve on the reference surface that joins twoconsecutive first steps together (thereby defining second steps as allthe steps encountered across the junction). Such junction lines can beproposed automatically to the user, and in examples the user may removejunction lines and/or add junction lines and/or perform corrections.

In parallel to this transition zones Versus constant offset zonesdefinition S20 (e.g. via junction lines), the method comprises defining(in the example of FIG. 1, by the user) a so-called convolution radius w(explained in details later), and which at this stage is merely adistance value related to the flattening (that is, a value thatdetermines the distance from the relevant ply boundary where theflattening is allowed, which degree of freedom is used to perform thetangent continuous connection). This can be entered via a prompt,possibly by the user validating or correcting a value proposed by thesystem. Be it precomputed by the system or provided ab initio by theuser, w is associated to a distance from ply boundaries where thematerial layers are affected by the flattening mentioned earlier. Inexample implementations discussed later, w is precisely this distance.Thus, at a distance w from such ply boundaries and parallel to such plyboundaries, are located respective boundary lines of the constant offsetsurfaces (i.e. thereby delimiting the first steps). The other boundaryline of a constant offset surface (i.e. the line parallel to the othercontour of the concerned first step), may be defined parallel to saidother contour (and thus can be considered parallel—but verticallyoffset, from the respective material layer thickness—to thecorresponding ply boundary), for example at the same distance w fromsaid contour (and this corresponds to the distance from the relevant plyboundary—projected on the reference surface—where molding material isdeposited above the first step).

Now, whether these data are predetermined, defined fully automatically,fully manually by the user, and/or by the uservalidating/correcting/adding to automatically precomputed propositions,the method may respect the following configuration (which is typical ofmost common composite parts). The distance between the (concerned)boundary line of a respective constant offset surface and the respectiveply boundary to which it is parallel (e.g. the convolution radius w inexamples) may be different or the same for all transition zones and/orfor both boundaries of each transition zone (e.g. in examples, it is thesame and equal to w, where w can be a value inferior to 20 mm, 10 mm,e.g. of the order 5 mm). In examples, the composite part has a (largest)length higher than 0.1 m, 0.5 m or 2 m, and/or lower than 20 m, 10 m or6 m. The composite parts comprises regions (at least 1, at least 2 or atleast 5) formed of a number of plies each time higher than 2, 3 or 5(typically of the order of 10). Each ply has a thickness of the order of0.1 mm, typically higher than 0.01 mm and lower than 0.2 mm. And inthese regions the plies form transition zones as explained earlier, witha width (e.g. lowest width) of the order of 5 mm. For each transitionzone, this distance may be at least larger than (or substantially equal)the width of the second steps of the respective transition zone, forexample of the order of the width of the second steps of the respectivetransition zone (e.g. in examples w is at least substantially equal tothe width of the second steps of the respective transition zone). Now,the width of said second steps of a respective transition zone is notnecessarily the same, such that any representative value (e.g. theaverage or the lowest value) may be retained. In case the same distanceis retained for all transition zones and second steps respective todifferent transition zones are not the same, similarly a representativevalue of all second step widths (e.g. the average or the lowest value)may be retained. This increases accuracy and/or relevancy of the design.In this configuration, the first steps may have a width (again, arepresentative value, e.g. average or lowest value, may be retained incase all first steps of the composite part do not have the same width,e.g. and statistically speaking) which is at least once and a half, ortwice, larger than the widths of the neighboring transition zones'second steps.

In an example, the junction lines comprise lines that join tworespective boundary lines at pairs of vertices which are close to eachother and/or at angles and/or zones of a curvature radius above apredetermined threshold of the respective boundary lines. Suchangle-joining junction lines may be detected automatically and thus(e.g. all) suggested to the user automatically. The method may run withsuch automatically detected junction lines, but other junction linesadded by the user may also be contemplated. As mentioned earlier, firststeps may be designated in another way (from defining junction lines),for example (e.g. automatically or semi-automatically) based on thewidth considerations mentioned earlier. These are however details ofimplementation, and the person skilled in the art of composite parts canrecognize how the designer or the algorithm can define at S26 therelevant junction lines. As a result, the output of S20 can beconsidered as predetermined data for the following discussions about therest of the method.

The method may at this point run an algorithm, based on a meshing of thereference surface (i.e. a mesh fitting the reference surface, withrespect to a predetermined error distance, called “sag value” as widelyknown). The mesh may be determined in any way, for example automatically(at S40 in the case of the example of FIG. 1), the resolution of themeshing (i.e. number of tiles per surface unit, thereby determining orbeing determined by the sag value) being for example a function of thevalue of the convolution radius w (e.g. such that two rows or more ofmesh tiles can be included between a contour and a parallel boundaryline). The mesh may be any type of 3D mesh, e.g. a quad mesh or atriangular mesh, providing vertices (i.e. 3D positions) connectedtwo-by-two by edges. The examples provided later are based on atriangular mesh in specific. The mesh may respect one or more differentconstraints. One constraint is that the contours of the ply map areformed by respective connected edges of the mesh. In other words, thecontours of the ply map are confounded with lines of the mesh (i.e.series of connected edges of the mesh). Another constraint is that otherrespective connected edges of the mesh are confounded in a similar waywith the boundary lines of the constant offset surfaces (as projected onthe reference surface). Yet another constraint is that yet otherrespective connected edges of the mesh are confounded in a similar waywith the junction lines. In conjunction with ƒ* mapping such edges inbijection (ƒ* defines a bijection from the connected edges to the image)with their corresponding 3D (ply boundaries, parallel boundary lines,and junctions between transition zone “ramps”), such constraints allowthe final surface to respect the initial topology, and thereby reach arelatively accurate and/or realistic result. The junction lines are anoption that allow an increase of accuracy in the shaping of thetransition zones, precisely by defining a regular border betweendifferent ramps of the transition zone.

The method of the example of FIG. 1 then comprises determining S50 afinal surface that corresponds to a tangent continuous connection of theconstant offset surfaces. As a result, the method provides a resultingfinal surface (that represents the outer surface) which featuresdifferent elements. The topology of the resulting surface featuresjunction lines and boundary edges of constant offset regions. Eachtopological face of the resulting surface is twice continuouslydifferentiable. Connections between faces of the resulting surface aretangent plane continuous. Curvature analysis may disclose a typicalconvolution regularized profile, as illustrated in FIG. 4. Some multiprocess can be activated. Surfaces underlying constant offset regionscan be procedural offset surfaces.

Examples of the method are now discussed in further details, withreference to FIGS. 5-33.

An example of a contemplated Resin Transfer Molding (RTM) process is nowdiscussed. The RTM process (as detailed in the book “Principles of themanufacturing of composite materials”, Suong V. Hoa, DEStechPublications, Inc., 2009, chapter 7, Liquid Composite Molding) isdedicated to manufacture composite parts. This process is particularlyused in the aerospace industry. Firstly, plies are laid up as a drystack of materials, named “fabric” or “preform” as illustrated in FIG.5. Then, this fabric is placed into a mold including of a lower matrixand an upper matrix. This mold is made of stiff metal. It is the countershape of the final composite part, as illustrated in FIG. 6. After theupper matrix is clamped over the lower matrix, the resin is injectedinto the cavity (inside which the fabric is confined) as illustrated inFIG. 7 by the arrow. Once all the fabric is wet out, the resin inletsare closed, and the laminate is allowed to cure. Both injection and curecan take place at either ambient or elevated temperature. Finally, themold is opened and the resulting composite part is removed, asillustrated in FIG. 8.

Prior to the industrial production, the actual shape of the resultingcomposite part is usually needed to machine the mold. A use of themethod is to compute this shape. It features the architecture of thecomposite part, defined by the ply map, as well as the overallsmoothness. This smoothness guaranties that the mold itself can bemachined by a cutting tool, that, when injected, resin easily flows intothe cavity, and the surface quality of the resulting part.

The following types of zone of the resulting geometry are particularlyimportant for mold quality. The first type is the “constant offsetsurface”. It is located at a constant distance from the referencesurface. The offset value is proportional to the number of stackedplies, as illustrated in FIG. 9, which shows constant offset surfaces 92and reference surface S, and the stacked plies in-between. The secondtype is the “transition surface” 102. It is the joining between twoconstant offset surfaces, as illustrated in FIG. 10. A transitionsurface is located at borders of plies and its height depends on thenumber of stacked plies. A last type of zone one can refer to as is theso-called “connection” 112 between a constant offset surface and atransition surface, as illustrated in FIG. 11 (which can also be seen asa part of the transition zone, or as the curve that borders a constantoffset zone and the neighboring transition zone, this being merely amatter of convention).

In an example, the input of the method is the ply map. It is the layoutof plies, defined on an input reference surface S, each ply beingassociated with an offset value. The ply map 122 and the referencesurface S are the topological and geometrical specifications of acomposite part, as shown on FIG. 12 which also illustrates contours 124of the ply map 122. The output of the method of the example is thevirtual shape 132 of the composite part manufactured from thesespecifications through the RTM process, as shown on FIG. 13. Thisvirtual shape can be used to define the numerical command for machiningthe mold matrices. It can be used as well to model the composite part inthe CAD system for digital mockup purpose. The ply map defines thelayout of plies, each ply being defined by an offset value and a closedcontour on the reference surface. In turn, this defines constant offsetzones and non-constant offset zones with respect to the referencesurface. Non-constant offset zones feature step-like discontinuousvariations. They are optionally localized by junction lines.

In an example, the first step of the method is to compute therestriction of constant offset zones that will remain invariant in theoutput shape. The shrink value of these invariant restriction zones maybe a user-defined radius. The second step of the example computes ameshing Σ of the reference surface S that is compliant with the topologyof the ply map and the invariant restrictions. Each triangle T isassociated with an offset value ƒ₀(T) corresponding to the cumulatedoffsets of stacked plies. This is the discontinuous offset variation ƒ₀.The third step of the example considers a change of the discontinuousoffset variation into a continuous offset variation ƒ by using the P1finite element interpolation (as explained for example in University ofAix-Marseille's R. Herbin's lecture textbook of Feb. 2, 2012, Chapter 4,“Elements finis de Lagrange”, which can be found at the website of theAix-Marseille Université, Centre de Mathématiques et d'Informatique,www.cmi.univ-mrs.fr/˜herbin/TELE/M1/chap4.pdf. The fourth step of theexample changes the step like transition zones into linear offsettransition zones by minimizing an energy criterion. This yields animproved offset variation ƒ*. The last step of the example defines thesmooth offset variation ƒ** as the convolution product of the offsetvariation ƒ* with a convolution kernel ϕ. The mathematical structure ofthe convolution kernel used in the method allows reaching anindustrially practical result. It involves the user-defined radius. Thepolynomial output mold surface is computed by approximating the surfaceobtained by applying the smooth offset variation ƒ**. This example isillustrated by the flowchart of FIG. 14.

In an example, the final surface (e.g. the mold surface) is computed asa variable offset of the reference surface. The variable offset of agiven surface is based on an offset variation as defined below. Theprinciple of the method of the example is to provide a smooth polynomialoffset variation on the reference surface by starting from adiscontinuous offset variation defined on a triangular meshing of thereference surface.

The concept of offset variation is now discussed.

Let M⊂

³ be a surface. An offset variation g on M is a mapping g:M→

. It is interpreted as follows. Given a point x∈M and N(x) the normalvector of surface M at point x, the corresponding offset point y isdefined by y=x+g(x)N(x), as illustrated in FIG. 15. The variable offsetsurface M′ of M defined by the offset variation g isM′={x+g(x)N(x),x∈M}.

Suppose now that surface M is a parameterized surface, meaning that M isthe image of a mapping S:[a,b]×[c,d]→

³, which is written M={S(u,v),(u,v)∈[a,b]×[c,d]}. The offset variation gon M defines an offset variation on [a,b]×[c,d] noted h:[a,b]×[c,d]→

by h(u,v)=g(S(u,v)). The offset point of (u,v) is S(u,v)+h(u,v)N(u,v)where

${N\left( {u,v} \right)} = \frac{{S_{u}\left( {u,v} \right)} \times {S_{v}\left( {u,v} \right)}}{{{S_{u}\left( {u,v} \right)} \times {S_{v}\left( {u,v} \right)}}}$is the normal vector of parameterized surface S. The variable offsetsurface of S defined by the offset variation h is parameterized byS′:[a,b]×[c,d]→

³ with S′(u,v)=S(u,v)+h(u,v)N(u,v).

Geometrical and numerical input data of examples of the method are nowdiscussed.

From the geometrical point of view, input data may be defined by thereference surface and by a list of closed contours C_(i), i=1, . . . ,n, on the reference surface, each contour being the boundary of a ply.Each contour C_(i) is associated with a thickness t_(i)>0.

Additional input data may be provided.

In addition to the group of plies and thicknesses as defined previously,the user may be asked to provide (e.g. or validate or correct/complete)the so-called “junction lines” mentioned earlier. Alternatively oradditionally, the user may simply designate first steps, or yet this canbe performed automatically in any way or be predetermined to the method.These lines define where the transition zones are located, separatingthe constant offset zones, that the algorithm is intended to smooth. Forexample, FIG. 16 illustrates a typical ply map with contours 124. FIG.17 illustrates the user-defined junction lines (bold lines 172 joiningangles 174).

Computing the parallel boundary lines of constant offset zones may thenbe performed automatically and easily. Junction lines define transitionzones. Constant offset zones are the remaining ones, as illustrated inFIG. 18. Parallel lines may be computed from boundary lines of constantoffset zones according to the following procedure. For each constantoffset zone, get the boundary line that does not belong to the outsideboundary. Parallel boundary line of this boundary line is computedtoward the inside of the constant offset zone. The distance between theboundary line and the parallel boundary line is the user-definedconvolution radius. FIG. 19 illustrates the parallel boundary lines(dotted lines 192) of constant offset zones. In addition, junction linesmay be updated by joining the corresponding vertices of parallelboundary lines. FIG. 20 illustrates the updated junction lines (boldlines). The initial ply map may thus be enriched with all parallelboundary lines and updated optional junction lines defined previously.This yields a topology that splits the reference surface into a bunch ofadjacent faces, as illustrated in FIG. 21. Each face of this topology isassociated (thanks to the input data and to each face corresponding to arespective step) with an offset value computed by cumulating offsetvalues of overlapping plies. FIG. 22 illustrates the cumulated offsetvalues over each region defined by plies overlapping.

An example of the computing of a compliant meshing Σ at S40 is nowdiscussed.

The split reference surface may now be meshed in such a way that thetriangles layout is compatible with the topology of adjacent faces,meaning that each triangle is associated with one face. The trianglesdensity is governed by a sag value. Traditionally, a too small sag valueleads to memory consumption and long computing time. Industrial testsprove that a good compromise is

$\frac{w}{100}$where w is the user-defined convolution radius (any value falling aroundthis value+/−50% can also be contemplated). Anyway, the sag value may belarger than 2.0×10⁻³ mm. The meshing disclosed in paper “L. A. Piegel,A. M. Richard, Tessellating trimmed NURBS surfaces, CAD, Vol. 27, No. 1,pp 16-26, 1995” may be appropriately used in the context of the method.FIG. 23 illustrates the local topology of adjacent faces. FIG. 24illustrates five faces 1, 2, 3, 4 and 5 of previous local topology. Thecompliant meshing of the local topology is illustrated in FIG. 25.Dotted lines are the polylines approximating faces boundary. Notice thatno triangle segment line features one vertex in a face and anothervertex in another face (in other words, each line of the topology is fitby the meshing with an uninterrupted line of connected edges of themesh, which is then mapped onto a corresponding curve in the finalsurface or stack of material layers). The resulting meshing is noted Σ.It should be understood that Σ remains unaffected all though thesubsequent process. It is not deformed, Σ is a read only data. It isused to define a sequence of smoother and smoother offset variationsƒ₀:Σ→

, ƒ:Σ→

, ƒ*:Σ→

and finally ƒ**:Σ→

as explained in the following.

In the following, it is provided an example of the method wherein thefinal surface corresponds to a convolution product between a function ƒ*defining a continuous connection of the constant offset surfaces thatinterpolates the ply boundaries (i.e. the ply boundaries belong, as aconstraint, to the image of function ƒ* on the mesh, e.g. theinterpolation being performed smoothly and/or monotonously between twointerpolations) and a convolution kernel ϕ defined for x∈

³ and of the type ϕ(x)=0 if ∥x∥>w, where ∥x∥ is the Euclidean norm of xand w>0 is the convolution radius. Specific examples which are efficientfor arriving at this mathematical result are presented. However, themethod may implement any other algorithm to arrive at the samemathematical result. For example, all values of functions ƒ₀ and ƒ arenot necessarily computed (at least entirely) as such. Indeed, only somevalues are needed and can be used as constraints of the later-definedenergy without the computations of the whole functions (the definitionof the functions standing in the following as a mere example, for thesake of clarity). Also, values of function ƒ need not necessarily becomputed. Function ƒ may indeed be merely an efficient starting pointfor the specific program used for the energy minimization (e.g.Gauss-Seidel), as it is close to the result ƒ* (being noted thatminimizations are often performed approximately by heuristics, as knownper se, and that good starting points are thus relevant for theefficiency of a minimization algorithm).

Discrete Discontinuous Offset Variation ƒ₀

In a first step of the example, the offset value ƒ₀(T) associated witheach triangle T of Σ is the one of the face it belongs to. This defineson Σ an offset variation ƒ₀:Σ→

that is not continuous at edges and vertices shared by trianglesequipped with distinct offset values. FIG. 26 illustrates the shape ofƒ₀ over a planar mesh.

Discrete Continuous Offset Variation ƒ

The previous discontinuous offset variation is made continuous by usingthe following method. The principle is to define the offset value ateach vertex and to compute offset values at other points byinterpolating offset values of vertices. This is the P1 finite elementstrategy.

Let x be a vertex and let Adj(x) be the set of triangles adjacent tothis vertex. The offset value ƒ(x) is computed as the maximum value ofits adjacent triangles offset values.

${f(x)}:={\max\limits_{T \in {{Adj}{(x)}}}\;{f_{0}(T)}}$

Let x be a point on Σ. Then, x necessarily belongs to a triangle T. Leta, b and c be the vertices of triangle T. The offset value ƒ(x)associated with point x is the convex combination of offset values atvertices a, b and c:ƒ(x):=α(x)ƒ(a)+β(x)ƒ(b)+γ(x)ƒ(c)

With

${\alpha(x)} = \frac{{Area}\left( {x,b,c} \right)}{{Area}\left( {a,b,c} \right)}$${\beta(x)} = \frac{{Area}\left( {a,x,c} \right)}{{Area}\left( {a,b,c} \right)}$${\gamma(x)} = \frac{{Area}\left( {a,b,x} \right)}{{Area}\left( {a,b,c} \right)}$

It should be noticed that if point x is shared by several triangles, theoffset value does not depend on the triangle chosen for the computation.This property makes ƒ:Σ→

continuous. FIG. 27 illustrates the shape of ƒ over a planar mesh.

Discrete Continuous and Energy Minimum Offset Variation ƒ*

Now, function ƒ* may correspond to a minimum of an energy that penalizesthe image of ƒ* over each face of the mesh deviating from a constantvalue. As the image of ƒ* over each face of the mesh may be an affinemapping, the energy may tend to render—globally—all such tile transformshorizontal, yet ensuring the connectivity constraint.

The offset variation ƒ:Σ→

is continuous over the mesh Σ. Nevertheless, the step-like shape oftransition zones is unsatisfactory. The best offset variation overtransition zones is as linear as possible, which is a result of thisexample. Vertices v_(i) of mesh Σ are divided into two sets: (1) fixedvertices and (2) free vertices. Vertices of constant offset zones arefixed. Vertices of ply boundaries are fixed. Vertices between constantoffset zones and the ply boundaries respective to the constant offsetzones (i.e. contours parallel to boundary liens of the constant offsetzones) are also fixed. Other vertices are free. This way, all freevertices belong to transition zones, but not all vertices of transitionzones are free, as illustrated in FIG. 28 (which extrapolates constantoffset zones 92 for additional fixed vertices at the constant offset).

Under appropriate renumbering, and for readability, free vertices arenoted v_(i) for i=1, . . . , n. Transition zones are improved byadjusting the offset value of the free vertices. So far, each vertex vof Σ is associated with an offset value ƒ(v). The unknowns of theregularization problem are offset values of free vertices, noted λ_(i)for i=1, . . . , n. Current values of λ_(i) are λ_(i)=ƒ(v_(i)) for i=1,. . . , n. Values ƒ(v_(i)) of fixed vertices v_(i), i>n are keptunchanged.

Once more, this optimization process is not used to deform mesh Σ. It isused to change the values of the offset variation ƒ that is defined onΣ.

Energy Criterion

The step-like improvement is performed in the example by minimizing anenergy criterion. This criterion is based on local versions of ƒassociated with each triangle T_(i) of Σ, and noted ƒ_(T) _(i) . Theenergy of each local ƒ_(T) _(i) is noted E(ƒ_(T) _(i) ) and the overallenergy E(ƒ) of ƒ is the sum of local energies:

${E(f)} = {\sum\limits_{i = 1}^{m}{E\left( f_{T_{i}} \right)}}$

Let T be an arbitrary triangle of Σ. The local ƒ_(T) and its energyE(ƒ_(T)) are defined as follows. Let a, b and c be the vertices oftriangle T. Firstly, an appropriate orthogonal axis system uvw isdefined such that points a, b and c are in plane uv. Let (u_(a),v_(a)),(u_(b),v_(b)) and (u_(c),v_(c)) the respective coordinates of points a,b and c in plane uv. Then, ƒ_(T) is the affine mapping ƒ_(T):

²→

such that ƒ_(T)(u_(a),v_(a))=ƒ(a), ƒ_(T)(u_(b),v_(b))=ƒ(b), andƒ_(T)(u_(c),v_(c))=ƒ(c), as illustrated in FIG. 29.

It can be uniquely written ƒ_(T)(u,v)=k₁u+k₂v+k₃ where scalarcoefficients k_(i) depend on (u_(a),v_(a)), (u_(b),v_(b)),(u_(c),v_(c)), ƒ(a), ƒ(b), and ƒ(c). The energy of ƒ_(T) is defined by:

${E\left( f_{T} \right)} = {\int\limits_{T}{{{\nabla{f_{T}\left( {u,v} \right)}}}^{2}{dudv}}}$

Since ƒ_(T) is an affine mapping, its gradient ∇ƒ_(T) does not depend on(u,v) so that previous integral is E(ƒ_(T))=Area(T)|∇ƒ_(T)|². Elementarycomputation shows that:

${E\left( f_{T} \right)} = {\frac{1}{{Area}(T)}{{{{f(a)}\left( {b - c} \right)} + {{f(b)}\left( {c - a} \right)} + {{f(c)}\left( {a - b} \right)}}}^{2}}$

Noting p(i), q(i) and r(i) the vertices indexes of triangle T_(i) andintroducing the notation λ_(i)=ƒ(v_(i)) of the unknowns, this can bewritten as:

${E\left( f_{T_{i}} \right)} = {\frac{1}{{Area}\left( T_{i} \right)}{{{\lambda_{p{(i)}}\left( {v_{q{(i)}} - v_{r{(i)}}} \right)} + {\lambda_{q{(i)}}\left( {v_{r{(i)}} - v_{p{(i)}}} \right)} + {\lambda_{r{(i)}}\left( {v_{p{(i)}} - v_{q{(i)}}} \right)}}}^{2}}$

It is clear that previous formula is a degree two polynomial withrespect to each variables λ_(p(i)), λ_(q(i)) and λ_(r(i)). Thispolynomial is noted:

${D_{i}\left( {X,Y,Z} \right)} = {\frac{1}{{Area}\left( T_{i} \right)}{{{X\left( {v_{q{(i)}} - v_{r{(i)}}} \right)} + {Y\left( {v_{r{(i)}} - v_{p{(i)}}} \right)} + {Z\left( {v_{p{(i)}} - v_{q{(i)}}} \right)}}}^{2}}$

Finally, the energy E(ƒ) is written introducing scalar unknowns λ_(i):

${E\left( {\lambda_{1},\ldots\;,\lambda_{n}} \right)} = {\sum\limits_{i = 1}^{m}{D_{i}\left( {\lambda_{p{(i)}},\lambda_{q{(i)}},\lambda_{r{(i)}}} \right)}}$

In the right hand expression of previous formula, symbols λ_(k) wherek>n are in fact fixed values ƒ(v_(k)).

Finally, the minimization problem may be written as:

$\min\limits_{{({\lambda_{1},\;\ldots\;,\lambda_{n}})} \in {\mathbb{R}}^{n}}\mspace{11mu}{E\left( {\lambda_{1},\ldots\;,\lambda_{n}} \right)}$

Gauss-Seidel minimization is an example solution for the minimizationproblem.

By construction, for each i=1, . . . , n the partial mapping λ_(i)→E(λ₁,. . . , λ_(n)) is a degree two polynomial in variable λ_(i). Theabscissa λ _(i) of its minimum value is the zero of its derivative,which is a straightforward computation thanks of degree two. Thealgorithm to minimize E(λ₁, . . . , λ_(n)) may be a Gauss-Seidel process(as described for example in Computing Discrete Minimal Surface andTheir Conjugates, Ulrich Pinkall, Konrad Poltier) based on thisproperty. Partial mappings are minimized iteratively until a stoppingcriterion ε=10⁻⁶ is reached.

This is illustrated by the following pseudo-code:

  (λ₁,...,λ_(n)):=(ƒ(ν_(i)),...,ƒ(ν_(n))) Repeat   e₀:=(λ₁,...,λ_(n)) For i:=1 to n do begin    Compute λ _(i) such that partial mapping   λ_(i)→E(λ₁,...,λ_(n)) is minimum    λ_(i):=λ _(i)  End for  e₁:=(λ₁,...,λ_(n)) Until |e₁ − e₀| ≤ ε

After convergence, the final parameters are noted λ*₁, . . . , λ*_(n).The offset variation mapping is now noted ƒ*:Σ→

and is defined by ƒ*(v_(i))=ƒ(v_(i)) if i>n and ƒ*(v_(i))=λ*_(i) if1≤i≤n. Values ƒ*(x) where x is not a vertex are computed according tothe P1 interpolation. FIG. 30 illustrates the shape of ƒ* over a planarmesh. Unchanged ƒ(v_(i)) are bold “X”.

Eventually, the method of the example considers a convolution productinvolving ƒ*.

The 1D convolution product is now discussed.

The convolution product (widely known and explained for example inConvolution, by Weisstein, Eric W, at the followingmathworld.wolfram.com) is a mathematical feature that has been used indifferent fields, such as signal processing or probability, and that canbe used in the context of the method as a tool for smoothing non-regularfunctions. Given two functions h:

→

and g:

→

such that their product hg:

→

is an integrable function, the convolution product of h and g is notedh*g and defined by:

${\left( {h*g} \right)(x)} = {\overset{+ \infty}{\int\limits_{- \infty}}{{h(t)}{g\left( {x - t} \right)}{dt}}}$

The method may make use of the convolution product as follows. Considerthe function ϕ:

→

named the “convolution kernel” and defined by

${\phi(x)} = {\frac{1}{k}\left( {w^{2} - x^{2}} \right)^{3}}$

if −w≤x≤w and ϕ(x)=0 otherwise, where w>0 is a constant parameter: theconvolution radius. Number

$k = {\underset{- w}{\int\limits^{+ w}}{\left( {w^{2} - x^{2}} \right)^{3}{dx}}}$

is a normalization constant designed in such a way that ∫_(−∞)^(+∞)ϕ(x)dx=1. FIG. 31A illustrates the curve x→ϕ(x).

Function ϕ is a tool function used to smooth non regular functions.Consider the discontinuous function g:

→

defined by g(x)=h>0 if x≥1 and g(x)=0 otherwise, illustrated by FIG.31B. Then, the convolution product g*ϕ is a regularization of g, meaningthat it saves its variations and that it features a smooth shape, asillustrated in FIG. 32. Notice that the extent of the transition zone[−w,w] between two constant zones is governed by parameter w.Furthermore, the average slope angle α of the transition zone is suchthat tan

$\alpha = {\frac{h}{2W}.}$

Now, the method may make use of a 3D convolution product in specific.

The convolution product is generalized to numerical functions defined on

^(n), but the example makes use of

³. The definition remains the same. Given h:

³→

and g:

³→

such that their product hg:

³→

is an integrable function, the convolution product of h and g is notedh*g and defined by:

${\left( {h*g} \right)(x)} = {\int\limits_{{\mathbb{R}}^{3}}{{h(t)}{g\left( {x - t} \right)}{dt}}}$

Where x=(x₁,x₂,x₃), t=(t₁,t₂,t₃) and dt=dt₁dt₂dt₃. Noting ∥x∥ theEuclidean norm of x∈

³, the convolution kernel ϕ is:

${\phi(x)} = {\frac{1}{k}\left( {w^{2} - {x}^{2}} \right)^{3}{dx}}$

if ∥x∥≤w and ϕ(x)=0 otherwise, where w>0 is the convolution radius. Thenormalization number is:

$k = {\int\limits_{\{{{x} < w}\}}{\left( {w^{2} - {x}^{2}} \right)^{3}{dx}}}$

By design, the first and second partial derivatives of this convolutionkernel are continuous, which is noted ϕ∈C²(

³).

An example of smoothing the 2D ½ discontinuous offset variation ƒ** isnow discussed.

The convolution product is designed to combine two functions defined onthe same source set

^(n). In the context of the example, the function to be regularized isthe offset variation of the reference surface ƒ*:Σ→

. Function ƒ* is not defined on

³, but on a mesh of the reference surface which is a subset Σ ⊂

³. Conversely, the convolution kernel ϕ is defined on

³. Consequently, the convolution definition may be adapted accordingly.For example, the method may define the regularized offset variation ƒ**:

³→

by:

${f^{**}(x)} = {\int\limits_{\sum}{{f^{*}(t)}{\phi\left( {x - t} \right)}{dt}}}$

Notice that ƒ** is defined on

³ despite ƒ* is defined on Σ ⊂

³.

An alternate definition could be ƒ**(x)=

h(t)ϕ(x−t)dt where h is an extrapolation of ƒ* defined on

³ and such that h(x)=ƒ*(x) for all x∈Σ. The method may define such anextrapolation and compute the volume integral resulting from thethree-dimensional convolution product h*ϕ.

Noting B(x,w) the w radius ball centered at point x, the formulabecomes:

${f^{**}(x)} = {{\int\limits_{\sum}{{f^{*}(t)}{\phi\left( {x - t} \right)}{dt}}} = {{\sum\limits_{i = 1}^{m}{\int\limits_{T_{i}}{{f^{*}(t)}{\phi\left( {x - t} \right)}{dt}}}} = {\underset{i \in {{{J{(x)}}T_{i}}\;\bigcap\;{B{({x,w})}}}}{\sum\mspace{11mu}\int}{f^{*}(t)}{\phi\left( {x - t} \right)}{dt}}}}$

Set J(x) is the subset of {1, . . . , m} that includes all triangleindexes i such that T_(i) intersects B(x,w). For computation purpose,identification of triangles T_(i), i∈J(x) may be achieved by using theteaching of paper “ERIT: A collection of efficient and reliableintersection tests, Martin Held, Journal of Graphics Tools, 2(4):25-44,1997”, and integral ∫_(T) _(i) _(∩B(x,w))ƒ*(t)ϕ(x−t)dt may be calculatedby using the teaching of paper “High Degree Efficient SymmetricalGaussian Quadrature Rules for the Triangle, D. A. Dunavant, Int. J. Num.Meth. Eng., 21, pp. 1129-1148 (1985)”. FIG. 33 illustrates the shape ofƒ** over a planar mesh.

If needed, derivating ƒ** may be performed easily. Partial derivativesof the smooth offset variation ƒ**(x) may notably be useful, for exampleto compute a polynomial approximation of ƒ** if wanted. The derivationis performed by switching the derivation operator and the integrationsign. For example, noting x=(x₁,x₂,x₃) the coordinates of x, thederivation of ƒ** with respect to x_(j) is:

${\frac{d}{{dx}_{j}}{f^{**}(x)}} = {{\frac{d}{{dx}_{j}}{\int\limits_{\sum}{{f^{*}(t)}{\phi\left( {x - t} \right)}{dt}}}} = {\int\limits_{\sum}{{f^{*}(t)}\frac{\partial}{\partial x_{j}}{\phi\left( {x - t} \right)}{dt}}}}$

It is evaluated by reusing the ball triangles intersection algorithm.

${\frac{d}{{dx}_{j}}{f^{**}(x)}} = {\underset{i \in {{{J{(x)}}T_{i}}\;\bigcap\;{B{({x,w})}}}}{\sum\mspace{11mu}\int}{f^{*}(t)}\frac{\partial}{\partial x_{j}}{\phi\left( {x - t} \right)}{dt}}$

The theoretical variable offset surface is now definable. So far, offsetvariation improvements are performed by using the mesh Σ of thereference surface. One can now define the offset variation with respectto the parameterization of the reference surface S. Given(u,v)∈[a,b]×[c,d], by definition S(u,v)∈

³. Let P_(Σ):

³→Σ be the projection on Σ in the following sense. Given x∈

³, P_(Σ)(x) is the point on Σ minimizing the distance |P_(Σ)(x)−x|.Then, the variable offset mapping on [a,b]×[c,d] is defined byφ:[a,b]×[c,d]→

with φ(u,v)=ƒ**(P_(Σ)(S(u,v))). In other words, S(u,v) is projected onthe mesh, yielding point P_(Σ)(S(u,v)), from which the variable offsetvalue ƒ**(P_(Σ)(S(u,v))) is computed.

The variable offset surface, which is the inner shape of the mold, isnow defined by the following theoretical parameterization. It is notedV:[a,b]×[c,d]→

³ withV(u,v)=S(u,v)+φ(u,v)N(u,v)

Where N(u,v) is the normal vector of reference surface S.

The formula for V(u,v) may be approximated if required (but this is notnecessary), for example to improve from the computational point of view.For example, the variable offset theoretical surface V(u,v) may beapproximated by a polynomial, as earlier-mentioned, which can be used tocreate a NURBS surface. This NURBS surface may be the persistent CADdata used by downstream applications, mainly digital mockup and moldmachining. This is however out of the scope of the present discussion.

The invention claimed is:
 1. A computer-implemented method for designingan outer surface of a composite part manufactured by molding a stack ofmaterial layers, each material layer having a respective thickness and arespective ply boundary, the method comprising: obtaining a referencesurface and a ply map that represents the stack of material layersbetween the reference surface and the outer surface, the ply map beingdefined as a list of contours defined on the reference surface, eachcontour being associated to the ply boundary of a respective materiallayer; defining constant offset surfaces, a constant offset surfacebeing a respective part of the outer surface which is to have a constantoffset value relative to the reference surface, the constant offsetvalue of a respective constant offset surface corresponding to the sumof the thicknesses of the material layers below the respective constantoffset surface; determining a final surface that corresponds to atangent continuous connection of the constant offset surfaces, eachconstant offset surface comprising at least a respective portion whichremains invariant in the final surface; and applying the determinedfinal surface in a 3D design of the composite part to be manufacturedusing a mold that physically corresponds to the determined finalsurface, wherein each transition surface corresponds to a convolutionproduct between a function ƒ* defining a continuous connection of theconstant offset surfaces that interpolates the ply boundaries and aconvolution kernel ϕ defined for x∈

³ and of the type ϕ(x)=0 if ∥x∥>w, where ∥x∥ is the Euclidean norm of xand w>0 is the convolution radius.
 2. The method of claim 1, wherein theconstant offset surfaces each have a respective boundary line parallelto the ply boundary of a respective material layer, the distance betweenthe boundary line of a respective constant offset surface and the plyboundary of the respective material layer being larger than orsubstantially equal to a step width between the respective constantoffset surface and the next and lower constant offset surface.
 3. Themethod of claim 1, wherein the convolution kernel is of the type${\phi(x)} = {\frac{1}{k}\left( {w^{2} - {x}^{2}} \right)^{3}}$ if∥x∥≤w, where k=∫_({∥x∥<w})(w²−∥x∥²)³dx.
 4. The method of claim 1,wherein function ƒ* is defined over a mesh fitting the reference surfaceand having faces, ƒ* being affine over each face of the mesh, thecontours of the ply map being formed by respective connected edges ofthe mesh, the respective edges being mapped in bijection by ƒ* onto theply boundaries.
 5. The method of claim 4, wherein other respectiveconnected edges of the mesh are mapped in bijection by ƒ* onto theboundary lines of the constant offset surfaces.
 6. The method of claim4, wherein other respective connected edges of the mesh are mapped inbijection by ƒ* onto junction lines, a junction line joining therespective boundary lines of two consecutive constant offset surfaces.7. The method of claim 4, wherein function ƒ* corresponds to a minimumof an energy that penalizes the image of ƒ* over each face of the meshdeviating from a constant value.
 8. A non-transitory computer readablestorage medium having recorded thereon a computer program comprisinginstructions for performing a method for designing an outer surface of acomposite part manufactured by molding a stack of material layers, eachmaterial layer having a respective thickness and a respective plyboundary, the method comprising: obtaining a reference surface and a plymap that represents the stack of material layers between the referencesurface and the outer surface, the ply map being defined as a list ofcontours defined on the reference surface, each contour being associatedto the ply boundary of a respective material layer; defining constantoffset surfaces, a constant offset surface being a respective part ofthe outer surface which is to have a constant offset value relative tothe reference surface, the constant offset value of a respectiveconstant offset surface corresponding to the sum of the thicknesses ofthe material layers below the respective constant offset surface;determining a final surface that corresponds to a tangent continuousconnection of the constant offset surfaces, each constant offset surfacecomprising at least a respective portion which remains invariant in thefinal surface; and applying the determined final surface in a 3D designof the composite part to be manufactured using a mold that physicallycorresponds to the determined final surface, wherein each transitionsurface corresponds to a convolution product between a function ƒ*defining a continuous connection of the constant offset surfaces thatinterpolates the ply boundaries and a convolution kernel ϕ defined forx∈

³ and of the type ϕ(x)=0 if ∥x∥>w, where ∥x∥ is the Euclidean norm of xand w>0 is the convolution radius.
 9. A system comprising a processorcoupled to a memory and a graphical user interface, the memory havingrecorded thereon a computer program comprising instructions for causingthe processor to perform a method for designing an outer surface of acomposite part manufactured by molding a stack of material layers, eachmaterial layer having a respective thickness and a respective plyboundary, the processor being configured to: obtain a reference surfaceand a ply map that represents the stack of material layers between thereference surface and the outer surface, the ply map being defined as alist of contours defined on the reference surface, each contour beingassociated to the ply boundary of a respective material layer; defineconstant offset surfaces, a constant offset surface being a respectivepart of the outer surface which is to have a constant offset valuerelative to the reference surface, the constant offset value of arespective constant offset surface corresponding to the sum of thethicknesses of the material layers below the respective constant offsetsurface; determine a final surface that corresponds to a tangentcontinuous connection of the constant offset surfaces, each constantoffset surface comprising at least a respective portion which remainsinvariant in the final surface; and apply the determined final surfacein a 3D design of the composite part to be manufactured using a moldthat physically corresponds to the determined final surface, whereineach transition surface corresponds to a convolution product between afunction ƒ* defining a continuous connection of the constant offsetsurfaces that interpolates the ply boundaries and a convolution kernel ϕdefined for x∈

³ and of the type ϕ(x)=0 if ∥x∥>w, where ∥x∥ is the Euclidean norm of xand w>0 is the convolution radius.